Conservation of Energy

The most typical energy conservation problem:

A roller-coaster of mass of $10$ kg starts at the top of a hill with a height of $20$ m and begins to slide down the hill purely due to gravity. At the other side there is another hill of height of $15$ m. What is the final velocity of the roller-coaster in m/s? Use $10 m/s^2$ as $g$ and assume friction is negligible.

Enter your answer without units.